Substance abuse remains a national crisis that presents substantial health-care and socio-economic costs. Of the panoply of abused substances, amphetamine (AMPH) and AMPH-like psychostimulants are among the most highly abused and exert considerable toxicity. Moreover, a recent surge in the generation of novel AMPH- like congeners that circumvent legal prohibitions has precipitated a new, major public health concern. My preliminary findings suggest that Regulator of G Protein Signaling-12 (RGS12) may be a critical molecular component underlying the CNS mechanisms by which these drugs exert their effects. RGS12-null mice were found to have reduced hyperlocomotion to AMPH and 3,4-methylenedioxy-methamphetamine (MDMA), but not to methylphenidate nor cocaine. Each of these psychostimulants exerts their locomotor-activating and rewarding properties by elevating extracellular dopamine (DA) in the striatum, but each does so via differential actions at the dopamine transporter (DAT): AMPH and MDMA are known to induce dopamine (DA) release via transport reversal of DAT, whereas MP and COC work to antagonize DA uptake through DAT. Additionally, I have found that the maintenance of AMPH-induced locomotor sensitization following three weeks of drug cessation is reduced in RGS12-null mice. The effects of AMPH and AMPH-like psychostimulants are dependent on mesolimbic and nigrostriatal brain structures, such as the ventral tegmental area, substantia nigra pars compacta, dorsal and ventral striatum -- all regions where Rgs12 mRNA and RGS12 protein are robustly expressed. Since submission of my original fellowship request, I have now shown that RGS12 reduces AMPH-induced [3H]DA release in ventral striatum, but not dorsal striatum, despite normal DAT- dependent [3H]DA uptake in these regions. Moreover, AMPH-induced c-Fos expression in RGS12-null mice is attenuated specifically in the ventral striatum. These new preliminary data suggest that RGS12 is required for AMPH action and, mechanistically, RGS12 may selectively modulate DAT-mediated DA release processes in the mesolimbic system. This resubmission thus seeks to further define the role of RGS12 in modulating the effects of AMPH and AMPH-like psychostimulants to advance our understanding of the mechanisms by which these drugs elicit addictive potential. The first Aim is to evaluate the role of RGS12 in behavioral sensitization and rewarding properties of drugs of abuse, using locomotor sensitization and conditioned place preference paradigms, respectively. The second Aim is to determine the neuroanatomical locus and neurocircuitry underlying the function of RGS12 on responses to AMPH-like drugs, as well as establish whether perturbations in DAT or other monoaminergic transporters (i.e., serotonin and norepinephrine transporters) explain the selectively attenuated hyperlocomotor responses to DAT-mediated DA releasers. My overall goal is to validate RGS12 as a novel molecular target within the process of how substance abuse is established and thus contribute to the future discovery of new therapeutic agents to ameliorate the debilitating disease of addiction.